How much loft insulation is enough?

Everyone understands the need to insulate the loft but few people know how much is enough. The loft is second only to the walls in terms of the proportion of the heat lost at 25%, but it is by far the easiest to deal with.

I often get told that the house is “super-insulated” as the loft has 250mm of insulation (“that’s nearly 10 inches, you know”). Well, Building Regulations call for a maximum U-value in the roof of 0.16W/m2 and that generally needs 270mm of mineral wool insulation - and that is nearly 11 inches. By today’s standards even that is not enough. A house built to Passivhaus principles will have 450mm mineral wool insulation in the loft, giving a U-value of less 0.08 W/m2. In a retrofit aiming at something between 300mm and 450mm would be a good target.

Materials

The choice of insulating material will be dictated by a few factors; space, access and probably price.

Dealing first with space, rigid foam insulation (the PUR or PIR Kingspan or Celotex types) are about twice as good insulators as wool types (sheep wool, Rockwool, etc.) and so take half the space. Where we have discussed 300mm as a good target, getting the same level of insulation from PUR or PIR will take only 150mm.

Access is often a big issue, which is an argument in favour of sustainable insulation – sheep wool, hemp, cellulose – as these are much more pleasant to handle. In tight, confined, airless lofts moving rolls of mineral wool while wearing gloves, goggles and a mask can be irritating – literally and metaphorically. Natural insulation needs no such protective measures and is much more fun to play with.

But price may be a factor. Natural insulation is still more expensive and although it has many advantages it is still only as good an insulator as mineral wool.

Multifoil insulation can be an answer, but only a partial answer. The LABC (Local Authority Building Control) suggest that multifoils should only be used in conjunction with another form of insulation (basically because they are not as good as the manufacturers say they are). But they are good draught excluders and easy to fit.

Positioning

Generally insulation is laid horizontally on the ceiling joists and there are good technical reasons for this. Lofts are usually ventilated spaces – the ventilation is necessary to keep the roof timbers in good order. Insulating at the rafters rather than the ceiling joist could mean that the ventilation will increase the heat lost from the house and put the roof timbers in danger of rot.

Insulating at the rafters level can be done but it needs to be done carefully and in full knowledge of all that is going on in the loft.

And finally…

Don’t forget the CERT (Carbon Emissions Reduction Target) Scheme. The scheme provides grant funding to all householders to improve energy efficiency measures. Just phone your energy supplier and ask for a survey under the CERT scheme.